In an optical scanning apparatus such as a laser beam printer having a document reading function, writing and reading of information are usually performed using a single laser light source. For example, the following two methods have been proposed. One is a method for changing the optical path of laser light by changing the angle of a reflecting mirror positioned on the optical path (see U.S. Pat. No. 4,573,084). The other is a method for dividing light into reading-use light and writing-use light by positioning a half mirror on the optical path (see U.S. Pat. No. 4,386,374).
First, with the method for changing the angle of the reflecting mirror, as illustrated in FIG. 8, laser light is first deflected when reflected by a rotating polygon mirror 61. The deflected laser light goes through an f.theta. correction lens 62 and reaches a movable reflecting mirror 63. When the reflecting mirror 63 is in a position indicated by the solid line of FIG. 8, the laser light which has been modulated according to a predetermined data signal is guided to a surface of a photoreceptor 64 for writing information. On the other hand, when the reflecting mirror 63 is in a position indicated by the alternate long and two short dashes line of FIG. 8, the laser light illuminates a surface of a document 65. Information is read out when reflected light from the document 65 is received by a light receiving element 66.
With the method in which a half mirror is positioned on the optical path, as illustrated in FIG. 9, the laser light is deflected by the rotating polygon mirror 61, goes through the f.theta. correction lens 62 like in the above-mentioned method, and reaches a half mirror 67. The half mirror 67 divides the laser light into light which goes through the half mirror 67 and light which is reflected by the half mirror 67. The laser light reflected by the half mirror 67 illuminates the surface of the document 65. When reflected light from the document 65 is received by the light receiving element 66, information is read out. When reading information, a douser 68 is disposed between the photoreceptor 64 and the half mirror 67 so as to stop the laser light which has passed through the half mirror 67 from illuminating the photoreceptor 64. On the other hand, when writing information on the photoreceptor 64 using the laser light which has passed through the half mirror 67, the douser 68 is positioned outside the optical path of the laser light so that the laser light which has been modulated according to a predetermined data signal illuminates the photoreceptor 64.
However, with the optical scanning apparatuses employing the above-mentioned methods, although it is possible to write and read information with a single laser light source, it is impossible to simultaneously read information while writing information.
More specifically, with the method in which the angle of the reflecting mirror is changed, the laser light emitted by the laser light source illuminates the photoreceptor when writing information, and illuminates the document when reading information. Namely, the laser light illuminates either the photoreceptor or the document depending on the angle of the reflecting mirror.
Meanwhile, with the method in which the half mirror is positioned on the optical path, it is possible to illuminate both the photoreceptor and the document with the laser light. However, since the laser light used for writing information has been modulated according to the data signal, it can not be used for reading information. Additionally, since the laser light used for reading information is continuously emitted, it can not be used for writing information.
In short, the reading operation can not be performed simultaneously with the writing operation. Consequently, the reading and writing operations are executed in real time, and the processing efficiency is lowered.